Exogenous insulin is used in extremely low birth weight (ELBW) infants both to treat hyperglycemia and to improve substrate utilization with chronic parenteral nutrition. However, insulin's effect on protein metabolism is not well understood in this population, and the only study investigating amino acid kenetics suggests no anabolic effect but a marked increase in lactate concentration with insulin use. We propose studying the effect of insulin in the most common situations in which insulin is used in the ELBW population. These include: 1) early-onset hyperglycemia, 2) dexamethasone-induced hyperglycemia, and 3) as a nutritional adjuvant during parenteral nutrition.